1. Field of the Invention
The present invention relates to a solenoid-operated valve in which a movable iron core displaced under magnetic excitation of a solenoid is directly connected to a valve plug for opening/closing a fluid passage.
2. Description of the Related Art
Solenoid-operated valves have been hitherto used to control the flow direction of the compressed air by supplying the pressure fluid to an actuator or discharging the compressed air to the atmosphere.
For example, Japanese Laid-Open Utility Model Publication No. 60-178678 discloses a conventional solenoid-operated valve as described above. As shown in FIG. 16, the solenoid-operated valve 1 has an intake valve plug 5 for mutually switching the communication between first and second fluid ports 3, 4 of a valve body 2, and a movable iron core 8 provided with a seal member 7 for mutually switching the communication of a third fluid port 6 of the valve body 2 to function as a discharge valve. Two connecting rods 9a, 9b are inserted between the intake valve plug 5 and the movable iron core 8 in the valve body 2.
A solenoid 11 is arranged in a casing 10 disposed on an upper portion of the valve body 2. In the OFF state in which no current is supplied to the solenoid 11, the movable iron core 8 is displaced downwardly (in the direction of the arrow B) by a first spring 12. Therefore, a seal member 7 is seated on a first seat 13 to interrupt the communication between the third fluid port 6 and the inside of the solenoid-operated valve 1. Further, the valve plug 5 is displaced downwardly against the spring force of a second spring 14 by the movable iron core 8 pressing on the connecting rods 9a, 9b. As a result, the valve plug 5 is separated from a second seat 15, and the first fluid port 3 is communicated with the second fluid port 4.
When the current is supplied to the solenoid 11, then the movable iron core 8 is attracted toward a fixed iron core 16 under the magnetic excitation of an unillustrated coil, and the movable iron core 8 is displaced upwardly (in the direction of the arrow A) against the spring force of the first spring 12. Accordingly, the seal member 7 is separated from the first seat 13 and the third fluid port 6 is communicated with the inside of the solenoid-operated valve 1. Further, the valve plug 5, which is pressed by the connecting rods 9a, 9b, is seated on the second seat 15 in accordance with the spring force of the second spring 14, and the communication is interrupted between the first fluid port 3 and the second fluid port 4.
In the conventional solenoid-operated valve 1 shown in FIG. 16, the seal member 7 of the movable iron core 8 and the valve plug 5 are seated to effect the seal on the first and second seats 13, 15 in accordance with the spring forces of the first and second springs 12, 14, respectively. However, stable seating force is not obtained when the valve plug 5 presses the first and second seats 13, 15, and the response speed is not stable in opening/closing the valve plug, depending on the relationship between the respective spring forces of the first spring 12 and the second spring 14.
In order to improve the seating force on the second seat 15 on which the valve plug 5 is seated, it is necessary that the spring force of the second spring 14 is increased to press the valve plug 5 against the second seat 15. However, if the spring force of the second spring 14 is increased, the connecting rods 9a, 9b, which are inserted between the upper surface of the valve plug 5 and the lower surface of the movable iron core 8, are displaced upwardly together with the valve plug 5. Therefore, the seal member 7 is separated from the first seat 13 as the movable iron core 8 is semi-forcibly displaced upwardly when pressed by the connecting rods 9a, 9b. Therefore, it is impossible to retain the air-tightness between the third fluid port 6 and the inside of the solenoid-operated valve 1.
This problem may be solved by increasing the spring force of the first spring 12 in the same manner as in the second spring 14. However, it is difficult to adjust the balance between the spring forces of the first and second springs 12, 14. If the spring force of the first spring 12 is increased, it is necessary to increase the attracting force of the solenoid-operated valve 1, because the increased spring force acts as the resistance force when the movable iron core 8 is magnetically attracted upwardly. As a result, the electric power consumption of the solenoid-operated valve 1 is increased, and the entire apparatus of the solenoid-operated valve 1 should be large.
As shown in FIG. 17, for example, when a solenoid-operated valve 20 is used, a casing 23 is attached to an upper portion of a valve body 22 having a plurality of fluid ports 21a to 21c, and a solenoid 24 is arranged in the casing 23. The communication of the fluid ports 21a to 21c is magnetically switched by a valve mechanism 25 with the solenoid 24.
As shown in FIG. 18, a magnetic plate 28 of a magnetic material is interposed in a substantially horizontal direction between the valve body 22 and a bobbin 27 around which a coil 26 of the solenoid 24 is wound. The magnetic plate 28 has a substantially rectangular cross section. The inner circumferential surface 30 of the magnetic plate 28 opposed to the outer circumferential surface of the movable iron core 29 has a width dimension C.
When the current is supplied to the solenoid 24, the magnetic flux is generated under the magnetic excitation of the coil 26. The magnetic flux from the coil 26 goes through the inner circumferential surface 30 of the magnetic plate 28 toward the movable iron core 29, and then returns from the movable iron core 29 through the fixed iron core 31 to the coil 26. In this case, the magnetic flux is generated corresponding to the area of the inner circumferential surface 30 (see reference symbol C in FIGS. 17 and 18) of the magnetic plate 28 opposed to the movable iron core 29.
As a result, as shown in FIG. 17, the movable iron core 29 is attracted toward the fixed iron core 31, and thus the movable iron core 29 is displaced upwardly (in the direction of the arrow A). Further, a connecting member 32 and a valve plug 33 connected to the movable iron core 29 are displaced upwardly (in the direction of the arrow A). Accordingly, the fluid port 21a is communicated with the fluid port 21b. 
As shown in FIG. 18, in the conventional solenoid-operated valve 20, the magnetic flux generated from the coil 26 under the magnetic excitation of the coil 26 goes through the magnetic plate 28 toward the movable iron core 29. In this arrangement, it is demanded to increase the attracting force of the movable iron core 29 generated under the magnetic excitation of the coil 26 so that the response speed of the valve plug 33 is further improved.
On the other hand, the conventional solenoid-operated valve generally adopts a method in which a movable iron core is attracted and displaced toward a fixed iron core along an inner circumferential surface of a hole of a bobbin by magnetically exciting a solenoid (electromagnet), and a valve plug is operated together with the movable iron core.
However, in the conventional solenoid-operated valve, when the movable iron core is displaced along the inner circumferential surface of the hole of the bobbin under the magnetic excitation of the solenoid, the sliding surface of the movable iron core is in surface-to-surface contact with the inner circumferential surface of the bobbin, and sliding resistance is generated. Therefore, the response speed of the solenoid-operated valve is low with respect to the electric signal supplied from a controller or the like.
Further, the movable iron core slides on the bobbin while the sliding surface of the movable iron core is in surface-to-surface contact with the inner circumferential surface of the bobbin. Therefore, the sliding portions of the movable iron core and the bobbin are abraded, and durability is lowered. As a result, maintenance cycle is shortened.
Further, it is demanded to reduce the number of parts of the conventional solenoid-operated valve, for example, in order to realize a compact size and a light weight of the solenoid-operated valve. Additionally, the fixed iron core provided opposingly and coaxially with the movable iron core occupies a relatively large ratio in the cost of the entire solenoid-operated valve.